Magnetic powder and isotropic bonded magnet

ABSTRACT

Disclosed herein is a magnetic powder which can provide a magnet having excellent magnetic properties and having excellent reliability especially excellent stability. The magnetic powder is composed of an alloy composition represented by R x (Fe 1−y Co y ) 100−x−z−w B z Al w  (where R is at least one kind of rare-earth element, x is 7.1-9.9 at %, y is 0-0.30, z is 4.6-6.9 at %, and w is 0.02-1.5 at %), the magnetic powder being constituted from a composite structure having a soft magnetic phase and a hard magnetic phase, wherein the magnetic powder has magnetic properties characterized in that, when the magnetic powder is formed into an isotropic bonded magnet having a density ρ[Mg/M 3 ] by mixing with a binding resin and then molding it, the remanent magnetic flux density Br[T] at the room temperature satisfies the relationship represented by the formula of Br/ρ[x10 −6 T·m 3 /g]≧0.125; the irreversible susceptibility (χ irr ) of the isotropic bonded magnet which is measured by using an intersectioning point of a demagnetization curve in the J-H diagram representing the magnetic properties at the room temperature and a straight line which passes the origin in the J-H diagram and has a gradient (J/H) of −3.8×10 −6  H/m as a starting point is equal to or less than 5.0×10 −7  H/m; and the intrinsic coercive force (H CJ ) of the isotropic bonded magnet at the room temperature is in the range of 320-720 kA/m.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to magnetic powder and an isotropicbonded magnet. More particularly, the present invention relates tomagnetic powder and an isotropic bonded magnet which is produced, forexample, using the magnetic powder.

[0003] 2. Description of the Prior Art

[0004] For reduction in size of motors, it is desirable that a magnethas a high magnetic flux density (with the actual permeance) when it isused in the motor. Factors for determining the magnetic flux density ofa bonded magnet include magnetization of the magnetic powder and thecontent of the magnetic powder contained in the bonded magnet.Accordingly, when the magnetization of the magnetic powder itself is notsufficiently high, a desired magnetic flux density cannot be obtainedunless the content of the magnetic powder in the bonded magnet is raisedto an extremely high level.

[0005] At present, most of practically used high performance rare-earthbonded magnets use the isotropic bonded magnets which are made usingMQP-B powder manufactured by MQI Inc. as the rare-earth magnetic powderthereof. The isotropic bonded magnets are superior to the anisotropicbonded magnets in the following respect; namely, in the manufacture ofthe bonded magnet, the manufacturing process can be simplified becauseno magnetic field orientation is required, and as a result, the rise inthe manufacturing cost can be restrained. On the other hand, however,the conventional isotropic bonded magnets represented by thosemanufactured using the MQP-B powder involve the following problems.

[0006] (1) The conventional isotropic bonded magnets do not have asufficiently high magnetic flux density. Namely, because the magneticpowder that has been being used has poor magnetization, the content ofthe magnetic powder to be contained in the bonded magnet has to beincreased. However, the increase in the content of the magnetic powderleads to the deterioration in the moldability of the bonded magnet, sothere is a certain limit in this attempt. Moreover, even if the contentof the magnetic powder is somehow managed to be increased by changingthe molding conditions or the like, there still exists a limit to theobtainable magnetic flux density. For these reasons, it is not possibleto reduce the size of the motor by using the conventional isotropicbonded magnets.

[0007] (2) Although there are reports concerning nanocomposite magnetshaving high remanent magnetic flux densities, their coercive forces, onthe contrary, are so small that the magnetic flux densities (for thepermeance in the actual use) obtainable when they are practically usedin motors are very low. Further, these magnets have poor heat stabilitydue to their small coercive forces.

[0008] (3) The conventional bonded magnets have low corrosion resistanceand heat resistance. Namely, in these magnets, it is necessary toincrease the content of the magnetic powder to be contained in thebonded magnet in order to compensate the low magnetic properties(magnetic performance) of the magnetic powder. This means that thedensity of the bonded magnet becomes extremely high. As a result, thecorrosion resistance and heat resistance of the bonded magnet aredeteriorated, thus resulting in low reliability.

SUMMARY OF THE INVENTION

[0009] It is therefore an object of the present invention to providemagnetic powder that can produce a bonded magnet having excellentmagnetization and having excellent reliability especially excellenttemperature characteristics (that is, heat resistance and heatstability), and provide an isotropic bonded magnet formed from themagnetic powder.

[0010] In order to achieve the above object, the present invention isdirected to magnetic powder composed of an alloy composition representedby R_(x)(Fe_(1−y)Co_(y))_(100−x−z−w)B_(z)Al_(w) (where R is at least onekind of rare-earth element, x is 7.1-9.9 at %, y is 0-0.30, z is 4.6-6.9at %, and w is 0.02-1.5 at %), the magnetic powder being constitutedfrom a composite structure having a soft magnetic phase and a hardmagnetic phase, wherein the magnetic powder has magnetic propertiescharacterized in that:

[0011] when the magnetic powder is formed into an isotropic bondedmagnet having a density ρ[Mg/m³] by mixing with a binding resin and thenmolding it, the remanent magnetic flux density Br[T] at the roomtemperature satisfies the relationship represented by the formula ofBr/ρ[x10⁻⁶ T·m³/g]≧0.125;

[0012] the irreversible susceptibility (χ_(irr)) of the isotropic bondedmagnet which is measured by using an intersectioning point of ademagnetization curve in the J-H diagram representing the magneticproperties at the room temperature and a straight line which passes theorigin in the J-H diagram and has a gradient (J/H) of −3.8×10⁻⁶ H/m as astarting point is equal to or less than 5.0×10⁻⁷ H/m; and

[0013] the intrinsic coercive force (H_(CJ)) of the isotropic bondedmagnet at the room temperature is in the range of 320-720 kA/m.

[0014] According to the magnetic powder as described above, it ispossible to provide bonded magnets having excellent magnetic propertiesas well as excellent heat resistance (heat stability) and corrosionresistance.

[0015] In the present invention, it is preferred that when the magneticpowder is formed into an isotropic bonded magnet by mixing with abinding resin and then molding it, the absolute value of theirreversible flux loss (initial flux loss) is equal to or less than6.2%.

[0016] This makes it possible to provide bonded magnets havingespecially excellent heat resistance (heat stability).

[0017] In these cases, it is preferred that said R comprises rare-earthelements mainly containing Nd and/or Pr. This makes it possible toimprove saturation magnetization of the hard phase of the compositestructure (in particular, nanocomposite structure), and thereby thecoercive force is further enhanced.

[0018] Further, it is also preferred that said R includes Pr and itsratio with respect to the total mass of said R is 5-75%. This makes itpossible to improve the coercive force and rectangularity with less dropof the remanent magnetic flux density.

[0019] Further, it is also preferred that said R includes Dy and itsratio with respect to the total mass of said R is equal to or less than14%. This makes it possible to improve the coercive force and the heatresistance (heat stability) without markedly lowering the remanentmagnetic flux density.

[0020] In the present invention, it is also preferred that the magneticpowder is obtained by quenching the alloy of a molten state. Accordingto this, it is possible to refine the microstructure (crystallinegrains) relatively easily, thereby enabling to further improve themagnetic properties of the bonded magnet.

[0021] Further, it is also preferred that the magnetic powder isobtained by milling a melt spun ribbon of the alloy which ismanufactured by using a cooling roll. According to this, it is possibleto refine the microstructure (crystalline grains) relatively easily,thereby enabling to further improve the magnetic properties of thebonded magnet.

[0022] Furthermore, it is also preferred that the magnetic powder issubjected to a heat treatment for at least once during the manufacturingprocess or after its manufacture. According to this, homogeneity(uniformity) of the structure can be obtained and influence of stressintroduced by the milling process can be removed, thereby enabling tofurther improve the magnetic properties of the bonded magnet.

[0023] In the magnetic powders described above, it is preferred that theaverage particle size lies in the range of 0.5-150 μm. This makes itpossible to further improve the magnetic properties Further, when themagnetic powder is used in manufacturing bonded magnets, it is possibleto obtain bonded magnets having a high content of the magnetic powderand having excellent magnetic properties.

[0024] Another aspect of the present invention is directed to anisotropic bonded magnet formed by binding a magnetic powder containingAl with a binding resin, wherein the isotropic bonded magnet ischaracterized in that:

[0025] when the density of the isotropic bonded magnet is defined asρ[Mg/m³], the remanent magnetic flux density Br[T] at the roomtemperature satisfies the relationship represented by the formula ofBr/ρ[x10⁻⁶ T·m³/g]≧0.125;

[0026] the irreversible susceptibility (χ_(irr)) of the isotropic bondedmagnet which is measured by using an intersect ioning point of ademagnetization curve in the J-H diagram representing the magneticproperties at the room temperature and a straight line which passes theorigin in the J-H diagram and has a gradient (J/H) of −3.8×10⁻⁶ H/m as astarting point is less than 5.0×10⁻⁷ H/m; and

[0027] the intrinsic coercive force (H_(CJ)) of the isotropic bondedmagnet at the room temperature is in the range of 320-720 kA/m.

[0028] According to the magnetic powder as described above, it ispossible to provide an isotropic bonded magnet having excellent magneticproperties as well as excellent heat resistance (heat stability) andcorrosion resistance.

[0029] In this isotropic bonded magnet, it is preferred that saidmagnetic powder is formed of R-TM-B—Al based alloy (where R is at leastone rare-earth element and TM is a transition metal containing Iron as amajor component thereof). This also makes it possible to provide anisotropic bonded magnet having particularly excellent magneticproperties as well as particularly excellent heat resistance (heatstability) and corrosion resistance.

[0030] Further, in this isotropic bonded magnet, it is also preferredthat the magnetic powder is composed of an alloy composition representedby R_(x)(Fe_(1−y)Co_(y))_(100−x−z−w)B_(z)Al_(w) (where R is at least onekind of rare-earth element, x is 7.1-9.9 at %, y is 0-0.30, z is 4.6-6.9at %, and w is 0.02-1.5 at %). This also makes it possible to provide anisotropic bonded magnet having particularly excellent magneticproperties as well as particularly excellent heat resistance (heatstability) and corrosion resistance.

[0031] Moreover, in this isotropic bonded magnet, it is also preferredthat said R comprises rare-earth elements mainly containing Nd and/orPr. This makes it possible to improve the coercive force of the bondedmagnet.

[0032] In this case, it is preferred that said R includes Pr and itsratio with respect to the total mass of said R is 5-75%. This makes itpossible to improve the coercive force and rectangularity with less dropof the remanent magnetic flux density.

[0033] Further, it is also preferred that said R includes Dy and itsratio with respect to the total mass of said R is equal to or less than14%. This makes it possible to improve the coercive force and the heatresistance (heat stability) without markedly lowering the remanentmagnetic flux density.

[0034] In the isotropic bonded magnets as described above, it ispreferred that the average particle size of the magnetic powder lies inthe range of 0.5-150 μm. This makes it possible to obtain an isotropicbonded magnet having a high content of the magnetic powder and havingexcellent magnetic properties.

[0035] Further, in the isotropic bonded magnets as described above, itis also preferred that the absolute value of the irreversible flux loss(initial flux loss) is equal to or less than 6.2%. This makes itpossible to provide particularly excellent heat resistance (heatstability).

[0036] Furthermore, in the isotropic bonded magnets as described above,it is also preferred that the magnetic powder is constituted from acomposite structure having a soft magnetic phase and a hard magneticphase. This improves magnetizability and heat resistance (heatstability), thus leading to less deterioration in the magneticproperties with elapse of time.

[0037] Preferably, the isotropic bonded magnets as described above areto be subjected to multipolar magnetization or have already beensubjected to multipolar magnetization. According to this, satisfactorymagnetization can be made even in the case where sufficient magnetizingmagnetic field is not obtained, thereby enabling to obtain sufficientmagnetic flux density.

[0038] Further, preferably, the isotropic bonded magnets as describedabove are used for a motor. By using the bonded magnet to motors, itbecomes possible to provide small and high performance motors.

[0039] These and other objects, structures and advantages of the presentinvention will be apparent from the following detailed description ofthe invention and the examples taken in conjunction with the appendeddrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0040]FIG. 1 is an illustration which schematically shows one example ofa composite structure (nanocomposite structure) of magnetic powderaccording to the present invention.

[0041]FIG. 2 is an illustration which schematically shows one example ofa composite structure (nanocomposite structure) of magnetic powderaccording to the present invention.

[0042]FIG. 3 is an illustration which schematically shows one example ofa composite structure (nanocomposite structure) of magnetic powderaccording to the present invention.

[0043]FIG. 4 is a perspective view showing an example of theconfiguration of an apparatus (melt spinning apparatus) formanufacturing a magnet material.

[0044]FIG. 5 is a sectional side view showing the situation in thevicinity of colliding section of the molten metal with a cooling roll inthe apparatus shown in FIG. 4.

[0045]FIG. 6 is a J-H diagram (coordinate) for explaining theirreversible susceptibility.

[0046]FIG. 7 is a J-H diagram (coordinate) that representsdemagnetization curves and recoil curves.

DETAILED DESCRIPTION OF THE INVENTION

[0047] In the following, embodiments of the magnetic powder and theisotropic bonded magnet according to this invention will be described indetail.

General Description of the Invention

[0048] At present, a magnet having high magnetic flux density ispractically required in order to reduce the size of motors or otherelectrical devices. In a bonded magnet, factors that determine themagnetic flux density are the magnetization of magnetic powder and thecontent (compositional ratio) of the magnetic powder contained in thebonded magnet. When the magnetization of the magnetic powder itself isnot so high, a desired magnetic flux density cannot be obtained unlessthe content of the magnetic powder in the bonded magnet is increased toan extremely high level.

[0049] As described in the above, the MQP-B powder made by MQI Inc.which is now being widely used can not provide sufficient magnetic fluxdensity depending on its use. As a result, in manufacturing the bondedmagnets, it is required to increase the content of the magnetic powderin the bonded magnet, that is, it is required to increase the magneticflux density. However, this in turn leads to the lack of reliability inthe corrosion resistance, heat resistance and mechanical strengththereof and the like. Further, there is also a problem in that theobtained magnet has a poor magnetizability due to its high coercivity.

[0050] In contrast, the magnetic powder and the isotropic bonded magnetaccording to this invention can obtain a sufficient magnetic fluxdensity and an adequate coercive force. As a result, without extremelyincreasing the content of the magnetic powder in the bonded magnet, itis possible to provide a bonded magnet having high strength and havingexcellent moldability, corrosion resistance and magnetizability. Thismakes it possible to reduce the size of the bonded magnet and increaseits performance, thereby contributing to reduction in size of motors andother devices employing magnets.

[0051] Further, the magnetic powder of the present invention may beformed so as to constitute a composite structure having a hard magneticphase and a soft magnetic phase.

[0052] While the MQP-B powder made by MQI Inc. is a single phasestructure of a hard magnetic phase, the magnetic powder of the presentinvention has the composite structure which has a soft magnetic phasewith high magnetization. Accordingly, it has an advantage that the totalmagnetization of the system as a whole is high. Further, since therecoil permeability of the bonded magnet becomes high, there is anadvantage that, even after a reverse magnetic field has been applied,the demagnetizing factor remains small.

Alloy Composition of Magnetic Powder

[0053] Preferably, the magnetic powder according to this invention isformed of R-TM-B—Al based alloys (where R is at least one rare-earthelement and TM is a transition metal containing Iron as a majorcomponent thereof). Among these alloys, an alloy having alloycompositions represented by R_(x)(Fe_(1−y)Co_(y))_(100−x−z−w)B_(z)Al_(w)(where R is at least one kind of rare-earth element, x is 7.1-9.9 at %,y is 0-0.30, z is 4.6-6.9 at %, and w is 0.02-1.5 at %) is particularlypreferred.

[0054] Examples of the rare-earth elements R include Y, La, Ce, Pr, Nd,Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, and a misch metal. In thisconnection, R may include one kind or two or more kinds of theseelements.

[0055] The content of R is set at 7.1-9. 9 at %. When the content of Ris less than 7.1 at %, sufficient coercive force cannot be obtained, andaddition of Al enhances the coercive force only to a small extent. Onthe other hand, when the content of R exceeds 9.9 at %, sufficientmagnetic flux density fails to be obtained because of the drop in themagnetization potential.

[0056] Here, it is preferable that R includes the rare-earth elements Ndand/or Pr as its principal ingredient. The reason for this is that theserare-earth elements enhance the saturation magnetization of the hardmagnetic phase which constitutes the composite structure (especially,nanocomposite structure), and are effective in realizing satisfactorycoercive force as a magnet.

[0057] Moreover, it is preferable that R includes Pr and its ratio tothe total mass of R is 5-75%, and more preferably 20-60%. This isbecause when the ratio lies within this range, it is possible to improvethe coercive force (coercivity) and the rectangularity by hardly causinga drop in the remanent magnetic flux density.

[0058] Furthermore, it is also preferable that R includes Dy and itsratio to the total mass of R is equal to or less than 14%. When theratio lies within this range, the coercive force can be improved withoutcausing marked drop in the remanent magnetic flux density, and thetemperature characteristic (such as heat stability) can be alsoimproved.

[0059] Cobalt (Co) is a transition metal element having propertiessimilar to Fe. By adding Co, that is by substituting a part of Fe by Co,the Curie temperature is elevated and the temperature characteristic ofthe magnetic powder is improved. However, if the substitution ratio ofFe by Co exceeds 0.30, both of the coercive force and the magnetic fluxdensity tend to fall off. The range of 0.05-0.20 of the substitutionratio of Fe by Co is more preferable since in this range not only thetemperature characteristic of the magnetic powder but also the magneticflux density thereof are improved.

[0060] Boron (B) is an element which is important for obtaining highmagnetic properties, and its content is set at 4.6-6.9 at %. When thecontent of B is less than 4.6 at %, the rectangularity of the B—H (J-H)loop is deteriorated. On the other hand, when the content of B exceeds6.9 at %, the nonmagnetic phase increase and the magnetic flux densitydrops sharply.

[0061] Aluminum (Al) is an element which is advantageous for improvingthe coercive force, and the effect of improvement of the coercive forceis conspicuous when its content lies in the range of 0.02-1.5 at %. Inaddition, the rectangularity and the maximum magnetic energy product arealso improved in this range in accompanying with the improvement in thecoercive force, and the heat resistance and corrosion resistance alsobecome satisfactory. In this connection, however, when the content of Ris less than 7.1 at %, these effects due to addition of Al are verysmall as described above. Further, when the content of Al exceeds 1.5 at%, the drop in the magnetization occurs. Further, another importanteffect obtained by containing 0.02-1.5 at % of Al is that theirreversible susceptibility (χ_(irr)) described hereinafter can be madesmall and the irreversible flux loss can be improved so that the heatresistance (heat stability) of the magnet is improved. In thisconnection, it is to be noted that if the amount of Al is less than 0.02at %, such effect is hardly realized and the effect for improvingcoercive force described above is small.

[0062] Of course, Al itself is a known substance. However, in thepresent invention, it has found through repeatedly conducted experimentsand researches that by containing 0.02-1.5 at % of Al to the magneticpowder constituted from a composite structure having a soft magneticphase and a hard magnetic phase, the following four effects arerealized, in particular these four effects are realized at the sametime, and this is the significance of the present invention.

[0063] (1) The coercive force of the magnetic powder can be improvedwhile maintaining the excellent rectangularity and the maximum magneticenergy product.

[0064] (2) The irreversible susceptibility (χ_(irr)) described below canbe made small.

[0065] (3) The irreversible flux loss can be improved, that is theabsolute value thereof can be lowered.

[0066] (4) Better corrosion resistance can be maintained.

[0067] As described above, the significant feature of the presentinvention resides in the addition of a trace amount of or a small amountof Al. In this regard, it is to be noted that addition of Al in theamount of more than 1.5 at % leading to an adverse effect, thus it isout of the scope of the present invention.

[0068] In this connection, the preferred range of the content of Al is0.02-1.5 at % as described above. In this case, it is more preferablethat the upper limit of the range is 1.0 at %, and it is the mostpreferable that the upper limit is 0.8 at %.

[0069] In addition, for the purpose of further improving the magneticproperties, at least one other element selected from the groupcomprising Cu, Si, Ga, Ti, V, Ta, Zr, Nb, Mo, Hf, Ag, Zn, P, Ge, Cr andW (hereinafter, referred to as “Q”) may be contained as needed. Whencontaining the element belonging to Q, it is preferable that the contentthereof should be equal to or less than 2 at %, and it is morepreferable that the content thereof lies within the range of 0.1-1.5 at%, and it is the most preferable that the content thereof lies withinthe range of 0.2-1.0 at %.

[0070] The addition of the element belonging to Q makes it possible toexhibit an inherent effect of the kind of the element. For example, theaddition of Cu, Si, Ga, V, Ta, Zr, Cr or Nb exhibits an effect ofimproving corrosion resistance.

Composite Structure

[0071] As described above, the magnetic material of the presentinvention has a composite structure having a soft magnetic phase and ahard magnetic phase.

[0072] In this composite structure (nanocomposite structure), a softmagnetic phase 10 and a hard magnetic phase 11 exist in a pattern(model) as shown in, for example, FIG. 1, FIG. 2 or FIG. 3, where thethickness of the respective phases and the particle diameter are on theorder of nanometers (for example, 1-100 nm). Further, the soft magneticphase 10 and the hard magnetic phase 11 are arranged adjacent to eachother (this also includes the case where these phases are adjacentthrough intergranular phase), which makes it possible to performmagnetic exchange interaction therebetween. In this regard, it is to benoted that the patterns illustrated in FIG. 1 to FIG. 3 are onlyspecific examples, and are not limited thereto. For example, the softmagnetic phase 10 and the hard magnetic phase 11 in FIG. 2 areinterchanged to each other.

[0073] The magnetization of the soft magnetic phase readily changes itsorientation by the action of an external magnetic field. Therefore, whenthe soft magnetic phase coexists with the hard magnetic phase, themagnetization curve for the entire system shows a stepped “serpentinecurve” in the second quadrant of the B—H diagram. However, when the softmagnetic phase has a sufficiently small size of less than several tensof nm, magnetization of the soft magnetic body is sufficiently andstrongly constrained through the coupling with the magnetization of thesurrounding hard magnetic body, so that the entire system exhibitsfunctions like a hard magnetic body.

[0074] A magnet having such a composite structure (nanocompositestructure) has mainly the following five features.

[0075] (1) In the second quadrant of the B—H diagram (that is, J-Hdiagram), the magnetization springs back reversively (in this sense,such a magnet is also referred to as a “spring magnet”).

[0076] (2) It has a satisfactory magnetizability, and it can bemagnetized with a relatively low magnetic field.

[0077] (3) The temperature dependence of the magnetic properties issmall as compared with the case where the system is constituted from ahard magnetic phase alone.

[0078] (4) The changes in the magnetic properties with the lapse of timeare small.

[0079] (5) No deterioration in the magnetic properties is observableeven if it is finely milled.

[0080] In the alloy composition described in the above, the hardmagnetic phase and the soft magnetic phase are respectively composed ofthe following, for instance.

[0081] The hard magnetic phase: R₂TM₁₄B system (where, TM is Fe or Feand Co), or R₂(TM, Al)₁₄B system (or R₂(TM, Q)₁₄B system, or R₂(TM, Al,Q)₁₄B system).

[0082] The soft magnetic phase: TM (α-Fe or α-(Fe, Co) in particular),or an alloy phase of TM and Al, a composite phase of TM and B, or acomposite phase of TM, B and Al (or these phases containing Q).

Manufacture of Magnetic Powders

[0083] As for the magnetic powders according to this invention, it ispreferable that they are manufactured by melt-spinning (quenching) amolten alloy, and more preferable that they are manufactured by millinga melt spun (quenched) ribbon obtained by quenching and solidifying themolten metal of the alloy. An example of such a method will be describedin the following.

[0084]FIG. 4 is a perspective view showing an example of theconfiguration of an apparatus (melt spinning apparatus) formanufacturing a magnet material by the melt spinning (quenching) methodusing a single roll, and FIG. 5 is a sectional side view showing thesituation in the vicinity of colliding section of the molten metal withthe cooling roll in the apparatus shown in FIG. 4.

[0085] As shown in FIG. 4, the melt spinning apparatus 1 is providedwith a cylindrical body 2 capable of storing the magnet material, and acooling roll 5 which rotates in the direction of an arrow 9A in thefigure relative to the cylindrical body 2. A nozzle (orifice) 3 whichinjects the molten metal of the magnet material alloy is formed at thelower end of the cylindrical body 2.

[0086] In addition, a heating coil 4 is arranged on the outer peripheryof the cylindrical body 2 in the vicinity of the nozzle 3, and themagnet material in the cylindrical body 2 is melted by inductivelyheating the interior of the cylindrical body 2 through application of,for example, a high frequency wave to the coil 4.

[0087] The cooling roll 5 is constructed from a base part 51 and asurface layer 52 which forms a circumferential surface 53 of the coolingroll 5.

[0088] The base part 51 may be formed either integrally with the surfacelayer 52 using the same material, or formed using a material differentfrom that of the surface layer 52.

[0089] Although there is no particular limitation on the material of thebase part 51, it is preferable that the base part 51 is formed of ametallic material with high heat conductivity such as copper or a copperalloy in order to make it possible to dissipate heat of the surfacelayer 52 as quickly as possible.

[0090] Further, it is preferable that the surface layer 52 is formed ofa material with heat conductivity equal to or lower than that of thebase part 51. Examples of the surface layer 52 include a metallic thinlayer of Cr or the like, a layer of metallic oxide and a ceramic layer.

[0091] Examples of the ceramics for use in the ceramic layer includeoxide ceramics such as Al₂O₃, SiO₂, TiO₂, Ti₂O₃, ZrO₂ Y₂O₃, bariumtitanate, and strontium titanate and the like; nitride ceramics such asAlN, Si₃N₄, TiN, and BN and the like; carbide ceramics such as graphite,SiC, ZrC, Al₄C₃, CaC₂, and WC and the like; and mixture of two or moreof these ceramics.

[0092] The melt spinning apparatus 1 is installed in a chamber (notshown), and it is operated preferably under the condition where theinterior of the chamber is filled with an inert gas or other kind ofgas. In particular, in order to prevent oxidation of a melt spun ribbon8, it is preferable that the gas is an inert gas such as argon gas,helium gas, nitrogen gas or the like.

[0093] In the melt spinning apparatus 1, the magnet material (alloy) isplaced in the cylindrical body 2 and melted by heating with the coil 4,and the molten metal 6 is discharged from the nozzle 3. Then, as shownin FIG. 5, the molten metal 6 collides with the circumferential surface53 of the cooling roll 5, and after the formation of a puddle 7, themolten metal 6 is cooled down rapidly to be solidified while draggedalong the circumferential surface 53 of the rotating cooling roll 5,thereby forming the melt spun ribbon 8 continuously or intermittently. Aroll surface 81 of the melt spun ribbon 8 thus formed is soon releasedfrom the circumferential surface 53, and the melt spun ribbon 8 proceedsin the direction of an arrow 9B in FIG. 4. The solidification interface71 of the molten metal is indicated by a broken line in FIG. 5.

[0094] The optimum range of the circumferential velocity of the coolingroll 5 depends upon the composition of the molten alloy, the wettabilityof the circumferential surface 53 with respect to the molten metal 6,and the like. However, for the enhancement of the magnetic properties, avelocity in the range of 1 to 60 m/s is normally preferable, and 5 to 40m/s is more preferable. If the circumferential velocity of the coolingroll 5 is too small, the thickness t of the melt spun ribbon 8 is toolarge depending upon the volume flow rate (volume of the molten metaldischarged per unit time), and the diameter of the crystalline grainstends to increase. On the contrary, if the circumferential velocity istoo large, amorphous structure becomes dominant. Further, in thesecases, enhancement of the magnetic properties can not be expected evenif a heat treatment is given in the later stage.

[0095] Thus obtained melt spun ribbon 8 may be subjected to at least oneheat treatment for the purpose of, for example, acceleration ofrecrystallization of the amorphous structure and homogenization of thestructure. The conditions of this heat treatment may be, for example, aheating in the range of 400 to 900° C. for 0.5 to 300 min.

[0096] Moreover, in order to prevent oxidation, it is preferred thatthis heat treatment is preferable to be performed in a vacuum or under areduced pressure (for example, in the range of 1×10⁻¹ to 1×10⁻⁶ Torr),or in a nonoxidizing atmosphere of an inert gas such as nitrogen gas,argon gas, helium gas or the like.

[0097] The melt spun ribbon (thin ribbon-like magnet material) 8obtained as in the above has a microcrystalline structure or a structurein which microcrystals are included in an amorphous structure, andexhibits excellent magnetic properties. The magnetic powder of thisinvention is obtained by milling thus obtained melt spun ribbon 8.

[0098] The milling method of the melt spun ribbon is not particularlylimited, and various kinds of milling or crushing apparatus such as ballmill, vibration mill, jet mill, and pin mill may be employed. In thiscase, in order to prevent oxidation, the milling process may be carriedout under vacuum or reduced pressure (for example, under a reducedpressure of 1×10⁻¹ to 1×10⁻⁶ Torr), or in a nonoxidizing atmosphere ofan inert gas such as nitrogen, argon, helium, or the like.

[0099] The average particle size of the magnetic powder is notparticularly limited. However, in the case where the magnetic powder isused for manufacturing isotropic bonded magnets described later, inorder to prevent oxidation of the magnetic powder and deterioration ofthe magnetic properties during the milling process, it is preferred thatthe average particle size lies within the range of 0.5 to 150 μm, morepreferably the range of 0.5 to 80 μm, and still more preferably therange of 1 to 50 μm.

[0100] In order to obtain a better moldability of the bonded magnet, itis preferable to give a certain degree of dispersion to the particlesize distribution of the magnetic powder. By so doing, it is possible toreduce the void ratio (porosity) of the bonded magnet obtained. As aresult, it is possible to raise the density and the mechanical strengthof the bonded magnet as compared with a bonded magnet having the samecontent of the magnetic powder, thereby enabling to further improve themagnetic properties.

[0101] Thus obtained magnetic powder may be subjected to a heattreatment for the purpose of, for example, removing the influence ofstress introduced by the milling process and controlling the crystallinegrain size. The conditions of the heat treatment are, for example,heating at a temperature in the range of 350 to 850° C. for 0.5 to 300min.

[0102] In order to prevent oxidation of the magnetic powder, it ispreferable to perform the heat treatment in a vacuum or under a reducedpressure (for example, in the range of 1×10⁻¹ to 1×x10 ⁻⁶ Torr), or in anonoxidizing atmosphere of an inert gas such as nitrogen gas, argon gas,and helium gas.

[0103] Thus obtained magnetic powder has a satisfactory bindability withthe binding resin (wettability of the binding resin). Therefore, when abonded magnet is manufactured using the magnetic powder described above,the bonded magnet has a high mechanical strength and excellent thermalstability (heat resistance) and corrosion resistance. Consequently, itcan be concluded that the magnetic powder is suitable for themanufacture of the bonded magnet.

[0104] In the above, the melt spinning (quenching) method is describedin terms of the single roll method, but the twin roll method may also beemployed. Besides, other methods such as the atomizing method which usesgas atomization, the rotating disk method, the melt extraction method,and the mechanical alloying method (MA) may also be employed. Since sucha melt spinning method can refine the metallic structure (crystallinegrains), it is effective for enhancing the magnetic properties,especially the coercive force or the like, of the bonded magnet.

Bonded Magnets and Manufacture thereof

[0105] Next, the isotropic bonded magnets (hereinafter, referred tosimply also as “bonded magnets”) according to this invention will bedescribed.

[0106] Preferably, the bonded magnets of this invention is formed bybinding the above described magnetic powder using a binding resin(binder).

[0107] As for the binder, either of a thermoplastic resin or athermosetting resin may be employed.

[0108] Examples of the thermoplastic resin include polyamid (example:nylon 6, nylon 46, nylon 66, nylon 610, nylon 612, nylon 11, nylon 12,nylon 6-12, nylon 6-66, nylon 6T and nylon 9T); thermoplastic polyimide;liquid crystal polymer such as aromatic polyester; poly phenylene oxide;poly phenylene sulfide; polyolefin such as polyethylene, polypropyleneand ethylene-vinyl acetate copolymer; modified polyolefin;polycarbonate; polymethylmethacrylate; polyester such as poly ethylenterephthalate and poly butylene terephthalate; polyether; polyetherether ketone; polyetherimide; polyacetal; and copolymer, blended body,and polymer alloy having at least one of these materials as a mainingredient. In this case, a mixture of two or more kinds of thesematerials may be employed.

[0109] Among these resins, a resin containing polyamide as its mainingredient is particularly preferred from, the viewpoint of especiallyexcellent moldability and high mechanical strength. Further, a resincontaining liquid crystal polymer and/or poly phenylene sulfide as itsmain ingredient is also preferred from the viewpoint of enhancing theheat resistance. Furthermore, these thermoplastic resins also have anexcellent kneadability with the magnetic powder.

[0110] These thermoplastic resins provide an advantage in that a widerange of selection can be made. For example, it is possible to provide athermoplastic resin having a good moldability or to provide athermoplastic resin having good heat resistance and mechanical strengthby appropriately selecting their kinds, copolymerization or the like.

[0111] On the other hand, examples of the thermosetting resin includevarious kinds of epoxy resins of bisphenol type, novolak type, andnaphthalene-based, phenolic resin, urea resin, melamine resin, polyester(or unsaturated polyester) resin, polyimide resin, silicone resin,polyurethane resin, and the like. In this case, a mixture of two or morekinds of these materials may be employed.

[0112] Among these resins, the epoxy resin, phenolic resin, polyimideresin and silicone resin are preferable from the viewpoint of theirspecial excellence in the moldability, high mechanical strength, andhigh heat resistance. In this case, the epoxy resin is especiallypreferable. These thermosetting resins also have an excellentkneadability with the magnetic powder and homogeneity (uniformity) inkneading.

[0113] The unhardened thermosetting resin to be used maybe either inliquid state or in solid (powdery) state at the room temperature.

[0114] The bonded magnet according to this invention described in theabove may be manufactured, for example, as in the following. First, themagnetic powder, a binding resin and an additive (antioxidant,lubricant, or the like) as needed are mixed and kneaded (warm kneading)to form a bonded magnet composite (compound). Then, thus obtained bondedmagnet composite is formed into a desired magnet form in a space freefrom magnetic field by a molding method such as compaction molding(press molding), extrusion molding, or injection molding. When thebinding resin used is a thermosetting type, the obtained green compactis hardened by heating or the like after molding.

[0115] In these three types of molding method, the extrusion molding andthe injection molding (in particular, the injection molding) haveadvantages in that the latitude of shape selection is broad, theproductivity is high, and the like. However, these molding methodsrequire to ensure a sufficiently high fluidity of the compound in themolding machine in order to obtain satisfactory moldability. For thisreason, in these methods it is not possible to increase the content ofthe magnetic powder, namely, to make the bonded magnet having highdensity, as compared with the case of the compaction molding method. Inthis invention, however, it is possible to obtain a high magnetic fluxdensity as will be described later, so that excellent magneticproperties can be obtained even without making the bonded magnet highdensity. This advantage of the present invention can also be extendedeven in the case where bonded magnets are manufactured by the extrusionmolding method or the injection molding method.

[0116] The content of the magnetic powder in the bonded magnet is notparticularly limited, and it is normally determined by considering thekind of the molding method and the compatibility of moldability and highmagnetic properties. More specifically, it is preferable to be in therange of 75-99.5 wt %, and more preferably in the range of 85-97.5 wt %.

[0117] In particular, in the case of a bonded magnet to be manufacturedby the compaction molding method, the content of the magnetic powdershould preferably lie in the range of 90-99.5 wt %, and more preferablyin the range of 93-98.5 wt %.

[0118] Further, in the case of a bonded magnet to be manufactured by theextrusion molding or the injection molding, the content of the magneticpowder should preferably lie in the range of 75-98 wt %, and morepreferably in the range of 85-97 wt %.

[0119] The density ρ of the bonded magnet is determined by factors suchas the specific gravity of the magnetic powder contained in the magnetand the content of the magnetic powder, and void ratio (porosity) of thebonded magnet and the like. In the bonded magnets according to thisinvention, the density ρ is not particularly limited to a specificvalue, but it is preferable to be in the range of 5.3-6.6 Mg/m³, andmore preferably in the range of 5.5-6.4 Mg/m³.

[0120] In this invention, since the magnetic flux density and thecoercive force of the magnetic powder are high, the bonded magnet formedfrom the magnetic powder provides excellent magnetic properties(especially, high maximum magnetic energy product (BH)_(max)) even whenthe content of the magnetic powder is relatively low. In this regard, itgoes without saying that it is possible to obtain the excellent magneticproperties in the case where the content of the magnetic powder is high.

[0121] The shape, dimensions, and the like of the bonded magnetmanufactured according to this invention are not particularly limited.For example, as to the shape, all shapes such as columnar, prism-like,cylindrical (ring-shaped), circular, plate-like, curved plate-like, andthe like are acceptable. As to the dimensions, all sizes starting fromlarge-sized one to ultraminuaturized one are acceptable. However, asrepeatedly described in this specification, the present invention isparticularly advantageous in miniaturization and ultraminiaturization ofthe bonded magnet.

[0122] Further, in view of the advantages described above, it ispreferred that the bonded magnet of the present invention is subject tomultipolar magnetization has been magnetized so as to have multipoles.

[0123] The bonded magnet of this invention as described in the above hasmagnetic properties in which the irreversible susceptibility (χ_(irr))which is measured by using an intersectioning point of a demagnetizationcurve in the J-H diagram (that is, coordinate where the longitudinalaxis represents magnetization (J) and the horizontal axis representsmagnetic field (H)) representing the magnetic characteristics at theroom temperature and a straight line which passes the origin in the J-Hdiagram and has a gradient (J/H) of −3.8×10⁻⁶ H/m as a starting point isequal to or less than 5.0×10⁻⁷ H/m, and the intrinsic coercive force(H_(cj)) of the magnet at the room temperature is in the range of320-720 kA/m. Hereinafter, explanation will be made with regard to therelationship among the irreversible susceptibility (χ_(irr)), theintrinsic coercive force (H_(CJ)), the remanent magnetic flux density(Br) and the density (ρ).

Irreversible Susceptibility (X_(irr))

[0124] As shown in FIG. 6, the irreversible susceptibility (χ_(irr)) isthe parameter which is represented by the following formula (unit isHenry/m, which is represented by H/m in this specification), wherein agradient of a tangential line of the demagnetization curve at a certainpoint P on the demagnetization curve in the J-H diagram is defined bydifferential susceptibility (X_(dif)) and a gradient of a recoil curvewhen the recoil curve from the point P is drawn with the demagnetizationfield being once reduced (that is, a gradient connecting the both endsof the recoil curve) is defined by reversible susceptibility (χ_(irr)).

Irreversible Susceptibility (χ_(irr))=differential susceptibility(X_(dif))−reversible susceptibility (X_(rev))

[0125] In this connection, it is to be noted that in the presentinvention, the point P is defined as an intersectioning point of thedemagnetization curve and the straight line y which passes the origin inthe J-H diagram and has a gradient (J/H) of −3.8×10⁻⁶ H/m.

[0126] In the present invention, the reason why the upper limit value ofthe irreversible susceptibility (χ_(irr)) at the room temperature isdefined as 5.0×10⁻⁷ H/m is as follows.

[0127] As stated in the above, the irreversible susceptibility (χ_(irr))represents the changing ratio of the magnetization, which is notreturned even if the absolute value thereof is reduced once afterdemagnetization field is applied, with respect to the magnetic field.Accordingly, by restraining the irreversible susceptibility (χ_(irr)) toa relatively small value, it is possible to improve heat stability ofthe bonded magnet and especially to reduce the absolute value of theirreversible flux loss. Actually, within the range of the irreversiblesusceptibility (χ_(irr)) of the present invention, the irreversible fluxloss obtained when the bonded magnet is being left in the atmosphere of100° C. for one hour and then the temperature is lowered into roomtemperature is equal to or less than 5% in its absolute value, whichmeans that practically satisfactory heat resistance (in particular whenused in motors or the like), that is satisfactory heat stability can beobtained.

[0128] In contrast, when the irreversible susceptibility (χ_(irr))exceeds 5.0×10⁻⁷ H/m, the absolute value of the irreversible flux lossis increased, so that it is not possible to obtain satisfactory heatstability. Further, since the intrinsic coercive force (H_(CJ)) islowered and the rectangularity thereof becomes poor, use of the obtainedbonded magnet is limited to the case where the permeance coefficient(Pc) becomes large (e.g. Pc≧5). Furthermore, the lowered coercive forcereduces the heat stability.

[0129] The reason why the irreversible susceptibility (χ_(irr)) at theroom temperature is defined as 5.0×10⁻⁷ H/m is described above. However,it is preferable that the value of the irreversible susceptibility(χ_(irr)) is as smaller as possible. Therefore, in the presentinvention, it is preferable that the irreversible susceptibility(χ_(irr)) is equal to or less than 4.5×10⁻⁷ H/m, and it is morepreferable that the irreversible susceptibility (χ_(irr)) is equal to orless than 4.0×10⁻⁷ H/m.

Intrinsic Coercive Force (H_(CJ))

[0130] It is preferred that the intrinsic coercive force (H_(CJ)) of thebonded magnet at room temperature is 320 to 720kA/m, and 400 to 640 kA/mis more preferable.

[0131] If the intrinsic coercive force (H_(CJ)) exceeds the above upperlimit value, magnetizability is deteriorated and therefore satisfactorymagnetic flux density can not be obtained.

[0132] On the other hand, if the intrinsic coercive force (H_(CJ)) islower than the lower limit value, demagnetization occurs conspicuouslywhen a reverse magnetic field is applied depending upon the usage of themotor and the heat resistance at a high temperature is deteriorated.Therefore, by setting the intrinsic coercive force (H_(CJ)) to the aboverange, in the case where the bonded magnet (cylindrical magnet inparticular) is subjected to multipolar magnetization, a satisfactorymagnetization can be accomplished even when a sufficiently highmagnetizing field cannot be secured, which makes it possible to obtain asufficient magnetic flux density, and to provide a high performancebonded magnet, especially a bonded magnet for motor.

Relationship between Remanent Magnetic Flux Density (Br) and Density (ρ)

[0133] In the present invention, it is preferred that the followingformula (I) is satisfied between the remanent magnetic flux densityBr(T) and the density ρ(Mg/M³).

0.125≦Br/ρ[x10⁻⁶ T·m ³/g]  (I)

[0134] In this connection, it is more preferable that the followingformula (II) is satisfied between the remanent magnetic flux densityBr(T) and the density ρ(Mg/m³), and it is most preferable that thefollowing formula (III) is satisfied therebetween.

0.128≦Br/ρ[x10⁻⁶ T·m ³/g]≦0.16  (II)

0.13Br/ρ[x10⁻⁶ T·m ³/g]≦0.155  (III)

[0135] When the value of Br/ρ[x10⁻⁶ T·m³/g] is less than the lower limitvalue of the formula (I), it is not possible to obtain a sufficientmagnetic flux density unless otherwise the density of the magnet isincreased, that is the content of the magnetic powder in the magnet isincreased. However, this in turn leads to problems in that availablemolding methods are limited, manufacturing cost is increased, andmoldability is lowered due to a reduced amount of the binding resin.Further, when a magnetic flux density of a certain level is to beobtained, a volume of the magnet is necessarily increased, which resultsin difficulty in miniaturizing devices such as motors.

Maximum Magnetic Energy Product (BH)_(max)

[0136] In the present invention, it is preferable that the maximummagnetic energy product (BH)_(max) of the bonded magnet is equal to orgreater than 60 kJ/m³, more preferably equal to or greater than 65kJ/m³, and most preferably in the range of 70 to 130 kJ/m³. When themaximum magnetic energy product (BH)_(max) is less than 60 kJ/m³, it isnot possible to obtain a sufficient torque when used for motorsdepending on the types and structures thereof.

Irreversible Flux Loss

[0137] In the present invention, it is preferable that the absolutevalue of the irreversible flux loss (that is, initial flux loss) isequal to or less than 6.2%, it is more preferable that it is equal to orless than 5.0%, and it is most preferable that it is equal to or lessthan 4%. This makes it possible to obtain a bonded magnet havingexcellent heat stability (heat resistance).

EXAMPLES

[0138] Hereinbelow, the actual examples of the present invention will bedescribed.

Example 1

[0139] Magnetic powders with alloy compositions(Nd_(0.7)Pro_(0.25)Dy_(0.05))_(8.5)Fe_(bal)Co_(7.0)B_(5.3)Al_(w) (thatis, various types of magnetic powders in which the content w of Al ischanged variously) were obtained by the method described below.

[0140] First, each of the materials Nd, Pr, Dy, Fe, Co, B and Al wasweighed, and then they were cast to produce a mother alloy ingot, and asample of about 15 g was cut out from the ingot.

[0141] A melt spinning apparatus 1 as shown in FIG. 4 and FIG. 5 wasprepared, and the sample was placed in a quartz tube 2 having a nozzle 3(circular orifice of which diameter is 0.6 mm) at the bottom. Afterevacuating the interior of a chamber in which the melt spinningapparatus 1 is housed, an inert gas (Ar gas) was introduced to obtain anatmosphere with desired temperature and pressure.

[0142] The cooling roll 5 of the melt spinning apparatus 1 is providedwith a surface layer 52 on the outer periphery of the base part 51 madeof Cu. The surface layer 52 is formed of ZrC and has a thickness ofabout 6 μm.

[0143] Then, the ingot sample in the quartz tube 2 was melted by highfrequency induction heating. Further, the jetting pressure (differencebetween the inner pressure of the quartz tube 2 and the pressure of theatmosphere) and the circumferential velocity were adjusted to obtain amelt spun ribbon.

[0144] Thus obtained melt spun ribbon was then coarsely crushed, and thepowder was subjected to a heat treatment in an argon gas atmosphere at690° C. for 300sec. In this way, the various types of magnetic powderseach having different contents w of Nb were obtained.

[0145] Next, for the purpose of adjustment of the particle size, eachmagnetic powder is milled by a milling machine in an argon gasatmosphere to obtain a magnetic powder having an average particle sizeof 60 μm.

[0146] To analyze the phase structure of the obtained magnetic powders,the respective magnetic powder was subjected to X-ray diffraction usingCu—Kα line at the diffraction angle of 200-60°. From the thus obtaineddiffraction pattern, the presence of diffracted peaks of a hard magneticphase, R₂(Fe Co) ₁₄B phase, and a soft magnetic phase, α-(Fe,Co) phase,were confirmed. Further, from the observation result using atransmission electron microscope (TEM), the formation of a compositestructure (nanocomposite structure) was confirmed in each magneticpowder.

[0147] A composite (compound) for bonded magnet was prepared by mixingthe respective magnetic powder with a polyamide resin (Nylon 12) and asmall amount of hydrazine antioxidant and lubricant, and then kneadingthem under the temperature of 225° C. for 15 min. In this case, thecompounding ratio (mixing ratio by weight) of the magnetic powder withrespect to the polyamide resin was common to the respective bondedmagnets. Specifically, in each of the bonded magnets, the content of themagnetic powder was about 97 wt %.

[0148] Then, each of the thus obtained compounds was crushed to begranular. Then, the granular substance(particle) was weighed and filledinto a die of a press machine, and then it was subjected to a compactionmolding (in the absence of a magnetic field) under the temperature of215° C. and the pressure of 750 MPa, to obtain an isotropic bondedmagnet of a columnar shape having a diameter of 10 mm and a height of 7mm. Next, the density ρ of each bonded magnet was measured by means ofthe Archimedes' method. The results thereof were shown in the Table 1.

Evaluation for Magnetic Properties and Irreversible Susceptibility(χ_(irr))

[0149] After pulse magnetization is performed for the respective bondedmagnets under the magnetic field strength of 3.2 MA/m, magneticproperties (remanent magnetic flux density Br, intrinsic coercive force(H_(CJ)), and maximum magnetic energy product (BH)_(max)) were measuredusing a DC recording fluxmeter (manufactured and sold by Toei IndustryCo. Ltd with the product code of TRF-5BH) under the maximum appliedmagnetic field of 2.0 MA/m. The temperature at the measurement was 23°C. (that is, room temperature).

[0150] As shown in FIG. 7, in the measured demagnetization curve of J-Hdiagram, a recoil curve having a starting point at an intersectioningpoint P between the demagnetization curve and a straight line whichpasses an origin and has a gradient of −3.8×10⁻⁶ H/m was produced withthe magnetic field being once changed to zero and then being returnedthe original state, and then the gradient of the recoil curve (that is,the gradient of the straight line connecting the both ends of the recoilcurve) was obtained and then it was defined as the reversiblesusceptibility (χ_(rev)). Further, the gradient of a tangential line ofthe demagnetization curve at the intersectioning point P was obtainedand then it was defined as the differential susceptibility (χ_(dif)) Theirreversible susceptibility (χ_(irr)) was obtained by the formula ofχ_(irr)=χ_(dif)−χ_(rev). The results of them are shown in the attachedTable 1.

Evaluation for Heat Resistance

[0151] Next, the heat resistance (heat stability) of each of the bondedmagnets (each having the column shape having the diameter of 10 mm andthe height of 7 mm) was examined. The heat resistance was obtained bymeasuring the irreversible flux loss (initial flux loss) obtained whenthe bonded magnet was being left in the atmosphere of 100° C. for onehour and then the temperature was lowered to the room temperature, andthen it was evaluated. The results thereof are shown in the attachedTable 1. In this connection, it is to be noted that smaller absolutevalue of the irreversible flux loss (initial flux loss) means moreexcellent heat resistance (heat stability).

Total Evaluation

[0152] As seen from the attached Table 1, the isotropic bonded magnets(No. 2 to No. 6) formed of the magnetic powders in which the content wof Al is 0.02 to 1.5 at % and the irreversible susceptibility (χ_(irr))is equal to or less than 5.0×10⁻⁷ H/m exhibit excellent magneticproperties (remanent magnetic flux density, intrinsic coercive force andmaximum magnetic energy product) and have small absolute value of theirreversible flux loss, so that the heat resistance of these magnets ishigh and the magnetizability thereof is excellent.

[0153] As described above, according to the present invention, it ispossible to obtain bonded magnets having high performance and highreliability (especially, heat resistance). In particular, these highperformances are exhibited when the bonded magnets are used in motors.

Example 2

[0154] In the same manner as Example 1, magnetic powders with alloycompositions(Nd_(0.75)Pr_(0.20)Dy_(0.05))_(8.6)Fe_(bal)Co_(6.9)B_(5.4)Al_(1.0) wereobtained by the method described below.

[0155] A composite (compound) for bonded magnet was prepared by mixingthe respective magnetic powder with a polyamide resin (Nylon 12) and asmall amount of hydrazine antioxidant and lubricant, and then kneadingthem under the temperature of 200-230° C. for 15 min. In this case, thecontent of the magnetic powder to be contained in each of the bondedmagnets was variously changed to obtain seven types of compounds.

[0156] Among thus obtained compounds, the compounds having a relativelyhigh content of the magnetic powder were crushed to be granular, andthen they were subjected to a compaction molding (in the absence of amagnetic field), while the compounds having a relatively low content ofthe magnetic powder were crushed to be granular, and then they weresubjected to an injection molding (in the absence of a magnetic field),thereby forming bonded magnets.

[0157] In this connection, it is to be noted that each bonded magnet wasformed into a columnar shape having a diameter of 10 mm and a height of7 mm.

[0158] Further, it is also to be noted that the compaction molding wascarried out by filing each granular substance (particle) into a die of apress machine and then it was subjected to a compaction molding underthe temperature of 210-220° C. and the pressure of 750 MPa. Further, theinjection molding was carried out under the conditions that the dietemperature at molding was 90° C. and the temperature inside theinjection cylinder was 230-280° C.

[0159] For each of thus obtained bonded magnets, magnetic propertiesthereof were measured and heat resistance thereof was also tested in thesame manner as the Example 1. The results thereof are shown in theattached Table 2.

Total Evaluation

[0160] As seen from the attached Table 2, the bonded magnets accordingto the present invention exhibit, over the wide range of the density ρ,excellent magnetic properties (remanent magnetic fluxdensity Br, maximummagnetic energy product (BH)_(max), and coercive force (H_(CJ))) andhave a small absolute value of the irreversible flux loss, so that theheat stability (heat resistance) of these magnets is also excellent.

[0161] In particular, the bonded magnets according to the presentinvention exhibit excellent magnetic properties even in the case wherethe bonded magnets are low density bonded magnets (that is, bondedmagnets having a small content of the magnetic powder) which can beobtained by means of an injection molding. The reason of this issupposed as follows.

[0162] When bonded magnets are low density, that is bonded magnets havea relatively large content of the binding resin, fluidity of thecompound during the kneading process or molding process is high. Thismakes it possible to knead the magnetic powder and the binding resin ata relatively low temperature within a short time period, so that it ispossible to easily accomplish that the magnetic powder and the bindingresin are uniformly mixed during the kneading process. Further, such ahigh fluidity of the compound makes it possible to easily carry out themolding at a relatively low temperature within a short time period. Inother words, molding conditions can be moderated. As a result, itbecomes possible to hold the deterioration (e.g. oxidization) of themagnetic powder during the kneading process and molding process at theminimum level, which results in production of bonded magnets having highmagnetic properties as well as improvement of the moldability.

[0163] Further, the high fluidity of the compound makes it possible tolower a void ratio of the obtained bonded magnets, so that mechanicalstrength and magnetic properties thereof are also improved.

Example 3

[0164] Using the magnetic powders obtained by Example 1, cylindrical(ring-shaped) isotropic bonded magnets having outer diameter of 22 mm,inner diameter of 20 mm and height of 4 mm were manufactured in the samemanner as Example 1. Then, thus obtained bonded magnets were subjectedto a multi-pole magnetization so as to have eight poles. At themagnetization process, an electric current of 16 kA was flowing througha magnetizing coil.

[0165] In this case, a magnitude of the magnetizing magnetic field forachieving 90% magnetizing ratio was relatively small, and this meansthat the magnetizability was excellent.

[0166] Further, using each bonded magnet as a magnet for a rotor, aspindle motor for CD-ROM drive was assembled. Then, each of the DCmotors was rotated at 1000 rpm to measure a back electromotive forcegenerated in the coil winding thereof. As a result, it has beenconfirmed that a voltage equal to or less than 0.80V can be obtained inthe motors using the bonded magnets of the samples No. 1 and No. 7(Comparative Examples), while a voltage equal to or greater than 0.96Vwhich is more than 20% higher value can be obtained in the motors usingthe bonded magnets of the samples No. 2 to No. 6 (Example of presentinvention).

[0167] With this result, it has found that it is possible to manufacturehigh performance motors by using the bonded magnets of the presentinvention.

[0168] In addition to the above, bonded magnets same as those ofExamples 1 to 3 were manufactured excepting that they are formed bymeans of an extrusion molding (the content of the magnetic powder ineach bonded magnet was 92 to 95 wt %). Then, the performance of thesebonded magnets were examined. As a result, it has found that the sameresults can be obtained by the motors using the bonded magnets

[0169] Further, bonded magnets same as those of Examples 1 to 3 weremanufactured excepting that they are formed by means of an injectionmolding (the content of the magnetic powder in each bonded magnet was 90to 93 wt %). Then, the performance of these bonded magnets wereexamined. As a result, it has found that the same results can beobtained by the motors using the bonded magnets

[0170] As described above, according to the present invention, thefollowing effects can be obtained.

[0171] Since each of the magnetic powders contains a predeterminedamount of Al and has a composite structure having a soft magnetic phaseand a hard magnetic phase, they have high magnetization and exhibitexcellent magnetic properties. In particular, intrinsic coercive forceand rectangularity thereof are improved.

[0172] The absolute value of the irreversible flux loss is small andexcellent heat resistance (heat stability) can be obtained.

[0173] Because of the high magnetic flux density that can be secured bythis invention, it is possible to obtain a bonded magnet with highmagnetic performance even if it is isotropic. In particular, sincemagnetic properties equivalent to or better than the conventionalisotropic bonded magnet can be obtained with a magnet of smaller volumeas compared with the conventional isotropic bonded magnet, it ispossible to provide a high performance motor of a smaller size.

[0174] Moreover, since a higher magnetic flux density can be secured, inmanufacturing a bonded magnet sufficiently high magnetic performance isobtainable without pursuing any means for elevating the density of thebonded magnet. As a result, the dimensional accuracy, mechanicalstrength, corrosion resistance, heat resistance (heat stability) and thelike can be improved in addition to the improvement in the moldability,so that it is possible to readily manufacture a bonded magnet with highreliability.

[0175] Since the magnetizability of the bonded magnet according to thisinvention is excellent, it is possible to magnetize a magnet with alower magnetizing field. In particular, multipolar magnetization or thelike can be accomplished easily and surely, and further a high magneticflux density can be obtained.

[0176] Since a high density is not required to the bonded magnet, thepresent invention is adapted to the manufacturing method such as theextrusion molding method or the injection molding method by whichmolding at high density is difficult as compared with the compactionmolding method, and the effects described in the above can also berealized in the bonded magnet manufactured by these molding methods.Accordingly, various molding method can be selectively used and therebythe degree of selection of shape for the bonded magnet can be expanded.

[0177] Finally, it is to be understood that the present invention is notlimited to Examples described above, and many changes or additions maybe made without departing from the scope of the invention which isdetermined by the following claims. TABLE 1 Irreversible ρ Br H_(cJ)(BH)_(max) Br/ρ χ_(Irr) Flux Loss Sample No. W (Mg/m³) (T) (kA/m)(kJ/m³) (×10⁻⁶T · m³/g) (×10⁻⁷ H/m) (%) 1 (Comp. Ex.) 0 6.27 0.82 31970.7 0.131 7.7 −7.0 2 (This Invention) 0.04 6.26 0.87 390 100.1 0.1394.9 −5.2 3 (This Invention) 0.1 6.32 0.90 462 108.3 0.142 4.2 −4.5 4(This Invention) 0.2 6.29 0.91 480 111.1 0.145 3.5 −3.9 5 (ThisInvention) 0.5 6.30 0.88 503 106.9 0.140 3.0 −3.5 6 (This Invention) 1.56.33 0.83 542 93.7 0.131 2.7 −3.1 7 (Comp. Ex.) 2.2 6.31 0.76 538 76.20.120 3.8 −4.0

[0178] TABLE 2 Irreversible Kneading Molding Molding ρ Br H_(cJ)(BH)_(max) Br/ρ χ_(Irr) Flux Loss Sample No. Temp. (° C.) Method Temp.(° C.) (Mg/m³) (T) (kA/m) (kJ/m³) (×10⁻⁶T · m³/g) (×10⁻⁷ H/m) (%)  8(This Invention) 220 Injection 230 5.30 0.78 532 80.4 0.147 2.2 −2.4Molding  9 (This Invention) 203 Injection 245 5.50 0.80 521 85.2 0.1452.5 −2.7 Molding 10 (This Invention) 211 Injection 260 5.67 0.82 51389.8 0.144 2.8 −3.0 Molding 11 (This Invention) 216 Injection 275 5.800.83 506 93.2 0.143 2.9 −3.2 Molding 12 (This Invention) 220 Compaction210 5.95 0.84 501 97.3 0.141 3.1 −3.5 Molding 13 (This Invention) 224Compaction 215 6.21 0.87 495 105.2 0.140 3.7 −4.0 Molding 14 (ThisInvention) 230 Compaction 220 6.48 0.90 481 113.4 0.139 4.4 −4.6 Molding

What is claimed is:
 1. Magnetic powder composed of an alloy compositionrepresented by R_(x)(Fe_(1−y)Co_(y))_(100−x−z−w)B_(z)Al_(w) (where R isat least one kind of rare-earth element, x is 7.1-9.9 at %, y is 0-0.30,z is 4.6-6.9 at %, and w is 0.02-1.5 at %), the magnetic powder beingconstituted from a composite structure having a soft magnetic phase anda hard magnetic phase, wherein the magnetic powder has magneticproperties characterized in that: when the magnetic powder is formedinto an isotropic bonded magnet having a density ρ[Mg/m³] by mixing witha binding resin and then molding it, the remanent magnetic flux densityBr[T] at the room temperature satisfies the relationship represented bythe formula of Br/ρ(x10⁻⁶ T·m³/g]≦0.125; the irreversible susceptibility(χ_(irr)) of the isotropic bonded magnet which is measured by using anintersectioning point of a demagnetization curve in the J-H diagramrepresenting the magnetic properties at the room temperature and astraight line which passes the origin in the J-H diagram and has agradient (J/H) of −3.8×10⁻⁶ H/m as a starting point is equal to or lessthan 5.0×10⁻⁷ H/m; and the intrinsic coercive force (H_(CJ)) of theisotropic bonded magnet at the room temperature is in the range of320-720 kA/m.
 2. The magnetic powder as claimed in claim 1, wherein whenthe magnetic powder is formed into an isotropic bonded magnet by mixingwith a binding resin and then molding it, the absolute value of theirreversible flux loss (initial flux loss) is equal to or less than6.2%.
 3. The magnetic powder as claimed in claim 1 or 2, wherein said Rcomprises rare-earth elements mainly containing Nd and/or Pr.
 4. Themagnetic powder as claimed in any one of claims 1 to 3, wherein said Rincludes Pr and its ratio with respect to the total mass of said R is5-75%.
 5. The magnetic powder as claimed in any one of claims 1 to 4,wherein said R includes Dy and its ratio with respect to the total massof said R is equal to or less than 14%.
 6. The magnetic powder asclaimed in any one of claims 1 to 5, wherein the magnetic powder isobtained by quenching the alloy of a molten state.
 7. The magneticpowder as claimed in any one of claims 1 to 6, wherein the magneticpowder is obtained by milling a melt spun ribbon of the alloy which ismanufactured by using a cooling roll.
 8. The magnetic powder as claimedin any one of claims 1 to 7, wherein the magnetic powder is subjected toa heat treatment for at least once during the manufacturing process orafter its manufacture.
 9. The magnetic powder as claimed in any one ofclaims 1 to 8, wherein the average particle size of the magnetic powderlies in the range of 0.5-150μm.
 10. An isotropic bonded magnet formed bybinding a magnetic powder containing Al with a binding resin, whereinthe isotropic bonded magnet is characterized in that: when the densityof the isotropic bonded magnet is defined as ρ[Mg/m³], the remanentmagnetic flux density Br[T] at the room temperature satisfies therelationship represented by the formula of Br/ρ[x10⁻⁶ T·m³/g]≧0.125; theirreversible susceptibility (χ_(irr)) of the isotropic bonded magnetwhich is measured by using an intersectioning point of a demagnetizationcurve in the J-H diagram representing the magnetic properties at theroom temperature and a straight line which passes the origin in the J-Hdiagram and has a gradient (J/H) of −3.8×10⁻⁶ H/m as a starting point isless than 5.0×10⁻⁷ H/m; and the intrinsic coercive force (H_(CJ) ) ofthe isotropic bonded magnet at the room temperature is in the range of320-720 kA/m.
 11. The isotropic bonded magnet as claimed in claim 10,wherein said magnetic powder is formed of R-TM-B—Al based alloy (where Ris at least one rare-earth element and TM is a transition metalcontaining Iron as a major component thereof).
 12. The isotropic bondedmagnet as claimed in claim 10 or 11, wherein the magnetic powder iscomposed of an alloy composition represented byR_(x)(Fe_(1−y)Co_(y))_(100−x−z−w)B_(z)Al_(w) (where R is at least onekind of rare-earth element, x is 7.1-9.9 at %, y is 0-0.30, z is 4.6-6.9at %, and w is 0.02-1.5 at %).
 13. The isotropic bonded magnet asclaimed claim 11 or 12, wherein said R comprises rare-earth elementsmainly containing Nd and/or Pr.
 14. The isotropic bonded magnet asclaimed in any one of claims 11 to 13, wherein said R includes Pr andits ratio with respect to the total mass of said R is 5-75%.
 15. Theisotropic bonded magnet as claimed in any one of claims 11 to 14,wherein said R includes Dy and its ratio with respect to the total massof said R is equal to or less than 14%.
 16. The isotropic bonded magnetas claimed in any one of claims 10 to 15, wherein the average particlesize of the magnetic powder lies in the range of 0.5-150 μm.
 17. Theisotropic bonded magnet as claimed in any one of claims 10 to 16,wherein the absolute value of the irreversible flux loss (initial fluxloss) is equal to or less than 6.2%.
 18. The isotropic bonded magnet asclaimed in any one of claims 10 to 17, wherein the magnetic powder isconstituted from a composite structure having a soft magnetic phase anda hard magnetic phase.
 19. The isotropic bonded magnet as claimed in anyone of claims 10 to 18, wherein the isotropic bonded magnet is to besubjected to multipolar magnetization or has already been subjected tomultipolar magnetization.
 20. The isotropic bonded magnet as claimed inany one of claims 10 to 19, wherein the isotropic bonded magnet is usedfor a motor.